Laser Perimeter Alarm




Learn how to fortify your fortress no matter what the size with this ingenious myriad of a customizable laser grid. Once someone steps through and breaks the laser signal, then off goes a quite noticeable, piercing alarm siren. Guard your room, office or workshop from pesky invaders and use it to safeguard your most prized possessions from high-profile robotic creation to the last jelly filled donut!

For more similar projects, kits for this and other gadgets, and much more just go to Ocalon Electronics. If your having any problems with getting the circuit to work, or just general Q&A questions feel free to leave them here.

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Step 1: Materials/Supplies

Parts List Includes

1. A single 1000uF Capacitor
2. A 5K Trimpot (larger values will work)
3. CdS Photocell (Cadmium Sulfide Cell)
4. Some Perforated Board
5. A 9v battery and clip
6. The 2N3904 Transistor
7. Several Small mirrors
8. About 5-12VDC Piezo Siren (102dB)
9. Any General Laser (650nm 5mw)


8. An L7805 5v Regulator
9. The Project Case
10. A 5 - 9 volt Adapter

Step 2: How Does It Work?

The heart of the system is the sensor. Without it we would not be able to do what we want, which is to sense a break in the beam. The cadmium sulphide photocell works by changing its resistance depending on the amount of light striking its surface.

We'll be using this large resistance change (about 10k ohm in daylight to about 1M ohm in pitch dark) to switch on/off a transistor for detecting the dark (when the beam is broken).

They capacitor is used to run the buzzer for several seconds (depending on the size of the cap) even in the beam is broken for only a fraction of a second. To get the siren to go off longer, just use a bigger capacitor (1000 uF) or simply put more capacitors in series with the current one.

Step 3: Proto-Breadboard Walkthrough - If Needed!

Here is a step-by-step walkthrough of building the circuit on a breadboard to test and see that it is indeed working. The point of doing this is to swap out any faulty parts that might become a hassle to replace once soldered together in the circuit. You can either follow each step below or skip ahead to begin soldering it all together.

I wasn't able to transfer it here, but if you need it just click here.

Step 4: Begin With the Board

For easy, one page viewing of all the steps needed to create the board click here.

First begin with a piece of perfboard as seen in the second image, with about the dimensions of about 1" inch by 1.5" inches. There will be excess board left over but you can just cut it away to your satisfaction. Then insert the CdS (Cadmium Sulfide) cell.

Bend the leads back 90 degrees so that they are parallel with the board and make sure the sensor is positioned up at an angle to suit your design. Next insert the 5k ohm trimming potentiometer next to the CdS cell and bend the leads at a 90 degree angle and flush with the board. Then solder the end pin of the trimpot to one of the leads of the CdS cell.

Step 5: Add the Trimpot and Transistor

Now just simply bend together the remaining two leads of the trimpot (the other end and the center leads) together and solder them both together (as seen in the last two pictures of the row above). The next step will be to bend the second lead of the CdS cell off to the side. As you can see below, we chose to weave it through the perfboard holes for a more sophisticated feel.

Then you mount the transistor (in the 3rd picture above the flat end is facing the transistor) and bend the middle pin (the transistor's middle pin is referred to as the 'base'). Now make sure to solder the middle pin of the transistor to the two trimpot pins that are connected to each other (3rd/4th picture above). Now bend the transistor's right pin (the pins are based on a front-side view, i.e. looking at the flat portion of the casing that has its information printed on it) to meet with the remaining pin of the CdS cell, and solder them together as seen in the 4th image.

Step 6: In Goes the Capacitor and Buzzer

Now you need to put in the capacitor. The negative end (marked with the black stripe with a minus sign on it) will connect to the transistor's emitter pin (leftmost pin when looking at its front) and the positive lead will connect to the joint - middle transistor pin and the two trimpot pins (images 2 and 3 above).

Once that is completed do the exact same thing with the buzzer. The negative lead (the black wire) will connect to the negative lead of the capacitor and the positive lead (red wire) will connect to the other pin on the capacitor.

Step 7: Power Up!

Now comes the last step, solder the positive lead of your 9 volt battery connector (again positive will always be red) to the positive capacitor and buzzer pins. Then solder the negative battery connector lead to the CdS cell pin that's only connected to the right transistor pin.

And there you have it, a fully functioning, laser perimeter alarm! As you can see, The very last photo is with a laser beam fixated on the sensor so that the alarm doesn't go off!

If you have any problems with getting the circuit to work, or just general Q&A questions feel free to leave them here. For more similar projects, kits for this and other gadgets, and much more just go to Ocalon Electronics



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    111 Discussions


    7 years ago on Step 2

    the left arm for cd cell where to connect
    and the other on of 2N3904 Transistor (the down ones) where to connect too ?!
    please help


    7 years ago on Step 7

    i made this project but how i connect the thing in broad band i can not understand from the diagram is it in parallel or series plz reply


    9 years ago on Step 7

     Works great, but I have to use a massive array of caps to get it to run for any acceptable length of time.  One 1000uf cap sends it fading out very quickly, so I wired about 10 of them in a series, and now it buzzes for about 4-5 seconds before fading.  All these caps, however, caused a delay in the way the transistor switches, so if someone walks quickly through the laser, it may not trip.

    I think I may try using just one really big cap, instead of an array.  Maybe that will eliminate the delay.

    4 replies

    Reply 8 years ago on Step 7

    Just use my schematic, the alarm will stay on forever, as long as there is power and the relays don't fail, even though it is much more complex it is worth to use it.

    Alarm Unit Diagram.jpg

    8 years ago on Step 2

    I have connected everything according to the circuit, the first time the alarm kept ringing even if the laser was on, so i tried again and this time the alarm wont go off, not even i the darkness. Weezoh or anyone help plz i am new to these stuff.


    8 years ago on Introduction

    HI, tried soldering the circuit but the alarm dosent seem to go off in the dark. I tried checking if i had done something wrong but everything seems to be alright. Can someone help me plz.


    8 years ago on Introduction

    How weak of a laser beam can you use and still get this to work? Or is there any way to make the system more sensitive? Would a bright LED work if it was placed extremely close to to the photocell?


    9 years ago on Introduction

     Could this be done with an inferred lazer and inferred light detector ? so they don't see the beam at night? And would the laser wear out ater a while?

    1 reply

    Reply 8 years ago on Introduction

    Yes. I have an '83 book that explains how to do something like this, but I am looking for a normally open LASCR in place of the photocell.


    8 years ago on Step 2

    I was wondering how the capacitor keeps the alarm circuit to stay on because wont the base stop that cuurent once the beam is on the photoresistor and the current flows through the photoresistor again? If so, does the capacitor keep the transistor to stay on or something?